Lasers are devices that can produce intense narrowly divergent, substantially single wavelength (monochromatic), coherent light. Laser light of different wavelengths can be advantageously applied in many fields, including biological, medical, military, space, industrial, commercial, computer, and telecommunications.
Semiconductor lasers may utilize an active region, which may be formed with a homojunction (using similar materials), single or double heterojunction using dissimilar materials), or with a quantum well (“QW”) or quantum cascade region. The quantum well structure may be formed when a low energy bandgap semiconductor material is typically positioned between two large bandgap semiconductor materials.
In order for lasing to occur, a laser device typically has a resonant cavity and a gain medium to create population inversion. In highly efficient semiconductor lasers, population inversion generally occurs with the injection of electrical carriers into the active region, and the resonant cavity is typically formed by a pair of mirrors that surround the gain medium. The method of injection of carriers can be divided into electrical injection of carriers and optical pumping for injection of carriers. Electrical injection is generally performed by an electrical current or voltage biasing of the laser. Optical pumping typically uses incident radiation that will allow the formation of electrons and holes in the laser. Additionally these methods can be operated in a continuous wave (CW) pulsed, synchronous or asynchronous modes.
Two common types of semiconductor lasers are in-plane, also known as edge emitting or Fabry Perot lasers, and surface emitting also known as vertical cavity surface emitting lasers (“VCSELs”). Edge emitting lasers emit light from the edge of the semiconductor wafer. In addition the resonant cavity is typically formed with cleaved mirrors at each end of the active region. The second type of semiconductor laser, VCSELs, emits light normal to the surface of the semiconductor wafer. The resonant optical cavity of a VCSEL may be formed with two sets of distributed Bragg reflector mirrors located at the top and bottom of the laser structure. The fundamental wavelength that characterizes quantum well and quantum cascade lasers is determined primarily by the thickness, composition and material of the quantum well.
However these lasers are typically limited to primarily one wavelength, thus the ability to create one laser that can produce many different wavelengths controllably and accurately could be very useful for many applications, such as in the field of telecommunications. A tunable laser could also be very useful in high data rate telecommunications applications like dense wavelength division multiplexing, which may be used in fiber optic communications.
For simplicity and clarity of illustration, the drawing figures illustrate the general manner of construction, and descriptions and details of well-known features and techniques may be omitted to avoid unnecessarily obscuring the examples of the invention. Additionally, elements in the drawing figures are not necessarily drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of the examples presented. Like reference numerals are used to designate like parts in the accompanying drawings.